Magnetic inks are known formed of magnetic material dispersed in a liquid carrier.
The magnetic material is typically magnetite (Fe.sub.3 O.sub.4), .gamma.-Fe.sub.2 O.sub.3 and the like. The magnetic material, in extremely finely divided form of the order of submicron size, is more or less permanently suspended in a liquid carrier with the aid of dispersing agents, surfactants, and the like to form a colloidal magnetic fluid, typically referred to as a ferrofluid.
The liquid carrier employed is usually a non-aqueous solvent, often an organic solvent of the non-polar type. Examples of non-aqueous solvents usable in the preparation of ferrofluids are aliphatic hydrocarbons, such as heptane, decane, mineral oil, kerosene, and the like, halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene and the like, aromatic solvents such as benzene, toluene and the like, silicone oils, etc.
The dispersing aid is included in the ferrofluid formulation to prevent aggregation of magnetic material particles in the non-aqueous solvent, which could lead to flocculation and deposition out of suspension of magnetic material. The dispersing aid, which may be one or more surfactants, wetting agents and the like, is applied to coat the surfaces of the individual magnetic particles so as to form a coating around the individual magnetic particles to prevent agglomeration or flocculation due to attraction therebetween. Aliphatic carboxylic acids having about 8 to 24 carbon atoms are known as dispersing aids for magnetic material, such as magnetite, to be colloidally suspended in non-aqueous solvents. In this regard, see U.S. Pat. Nos. 3,531,413 and 3,764,540.
For certain applications of magnetic inks it is desirable to employ water as the carrier fluid for the magnetic material in place of the more commonly used non-aqueous solvents. The present invention is directed to the preparation of colloidal suspensions of magnetic material, such as magnetite, in an aqueous medium, and the ferrofluid so produced.
Accordingly, it is an object of the present invention to provide a method for forming a colloidal suspension of magnetic particles, particularly magnetite, in water.
Another object of this invention is to provide an aqueous based magnetic ink.
A further object of this invention is to provide a specific combination of dispersing aids which can be employed to colloidally disperse magnetite in water.